Surface profiling apparatus

ABSTRACT

A broad band surface profiling apparatus including a reference calibrator for calibrating the apparatus to compensate for surface features of the reference surface. A user is instructed to conduct calibration measurement operations using a calibration sample having a calibration surface to obtain calibration surface topography data for the calibration sample. At each calibration measurement operation, an image representing the calibration surface topography data is displayed to the user and the user has the option to accept or reject the calibration surface topography data represented by the displayed image. The reference calibrator has a surface topography data processor and a mean surface calculator for calculating mean surface topography data using the processed calibration surface topography data accepted by the user to provide reference surface features data. A reference surface features remover is provided for adjusting surface topography data obtained for a sample surface in accordance with the reference surface features data.

This invention relates to a surface profiling apparatus, in particularsurface profiling apparatus for determining surface profile data usinginterferometric techniques.

As discussed in a paper entitled “Profilometry with a Coherence ScanningMicroscope” by Byron S. Lee and Timothy C Strand published in AppliedOptics Volume 29, No. 26 10 September 1990 at pages 3784 to 3788, asmanufacturing tolerances have reduced, demands have increased on opticalmetrology techniques for improved lateral and vertical resolution.Conventional monochromatic interferometric surface profiling apparatusoffers good vertical resolution in the nanometre to Angstrom range butphase ambiguity limits the measurement range to phase shifts of lessthan 2π.

As discussed in the paper by Lee and Strand, these problems can beaddressed by the use of coherence scanning or broadband scanninginterferometry which can provide practical measurement ranges easilyexceeding hundreds of micrometres.

Coherence scanning or broadband scanning interferometry uses a standardinterferometer such as a Michelson interferometer with a broadbandspatially incoherent light source such as a quartz halogen lamp.Generally, but not necessarily, the broadband source will be a whitelight source. One of the sample surface whose profile is to bedetermined and the reference mirror of the interferometer is movedrelative to the other along a scan path to change the relative pathlength and a two-dimensional image sensor such as a CCD camera is usedto sense the resulting interference pattern which changes as the samplesurface and reference mirror are moved relative to one another.

Each sensing element or pixel of the image sensor senses the portion ofthe interference pattern for a corresponding region or surface pixel ofthe sample surface and, as the sample surface and the reference mirrorare moved relative to one another, the amount or intensity of lightreceived by the sensing element will vary in accordance with the changein the interference fringes. The intensity of light received from aregion of the sample surface will increase or decrease in amplitude independence upon the path length difference between the light paths fromthe reference mirror and the sensing surface and will have a coherencepeak or extremum (maximum or minimum amplitude) at the position of zeropath difference. Where different regions of the surface have differentrelative heights, then those different regions will have coherence peaksat different positions along the scan path. Accordingly, the relativepositions of the coherence peaks can be used to provide surface profiledata, that is data representing the relative height of the differentregions of the sample surface.

The reference mirror may have significant form, that is the referencemirror may not be optically flat and may have other surface featuressuch as surface roughness or texture, for example the reference mirrormay have marks and even scratches. It is therefore desirable tocalibrate the surface profiling apparatus to compensate for thesesurface features of the reference mirror. Moreover, due to environmentalchanges and the like, changes in the reference mirror surface featuresmay occur over time. Although these changes will be very small, thenature of the measurements being made is such that they will have asignificant effect on the measurement results. These changes mean thatcalibration of the surface profiling apparatus during manufacture is notsufficient and that it will be necessary from time to time for the userof the apparatus to calibrate the apparatus to compensate for thesurface features of the reference mirror.

In one aspect, the present invention provides surface profilingapparatus having reference calibration means that facilitate calibrationof the reference by a user.

In one aspect, the present invention provides surface profilingapparatus having reference calibration means that enable a user toaccept or reject reference topography data acquired during thecalibration procedures that so that the user can discard unsuitablereference topography data.

In one aspect, the present invention provides surface profilingapparatus having reference calibration means that enables the user tosee the effect of addition of successive reference images enabling theuser to decide how many reference images are required to enablesatisfactory calibration to compensate for surface features of thereference mirror (that is features of form, surface roughness ortexture, including any marks scratches or the like), so enabling theuser to minimise the time required for calibration.

Embodiments of the present invention will now be described, by way ofexample, with reference to the accompanying drawings, in which:

FIG. 1 shows a schematic block diagram of a surface profiling apparatususing a coherence scanning or broadband scanning interferometer having areference calibrator;

FIG. 2 shows a graph of intensity against position Z to illustrate theinterference fringes for a sample surface region around a coherence peakor interference region;

FIG. 3 shows a functional block diagram of surface profiling apparatus;

FIG. 4 shows a simplified side-elevational, part sectional view of thesurface profiling apparatus shown in FIG. 3 but excluding the controlapparatus;

FIG. 5 shows a functional block diagram of computing apparatus that maybe programmed to provide the control apparatus shown in FIG. 3;

FIG. 6 shows a functional block diagram of the reference calibratorshown in FIG. 1;

FIGS. 7 to 9 show flowcharts for illustrating operation of the surfaceprofiling apparatus shown in FIG. 1 during a calibration procedure toobtain reference surface features removal data;

FIG. 10 shows, very schematically, one example of a referencecalibration user interface that may be provided by the surface profilingapparatus to assist the user during the reference calibration procedure;

FIGS. 11 a, 11 b and 11 c show examples of images that may be displayedto the user on the user interface during a reference calibrationprocedure;

FIG. 12 shows an example of a user interface that may be displayed to auser to set up a measurement operation for an actual sample;

FIG. 13 shows a flowchart for illustrating operation of the surfaceprofiling apparatus shown in FIG. 1 to determine the surface topographyof a sample; and

FIG. 14 shows, very schematically, an example of another referencecalibration user interface that may be provided by the surface profilingapparatus to assist the user during the reference calibration procedure.

Referring now the drawings, FIG. 1 shows a simplified schematic blockdiagram of a surface profiling apparatus 1 which has a broadband orcoherence scanning interferometer system 2 and data processing andcontrol apparatus 3.

The broadband scanning interferometer system 2 is based on aconventional interferometer but, as discussed in the introduction,instead of having a monochromatic spatially coherent light source, thebroadband scanning interferometer system 2 has a broadband source 4which may be, for example, a white light source such as a quartz halogenlamp coupled to a regulated DC power supply having a light intensityoutput user control 400 provided in the form of a user-rotatable knob.

The broadband source 4 provides broadband light L which is split by abeam splitter (illustrated in FIG. 1 as a single beam splitting prism) 5into a first light beam which is directed along a reference path RPtowards a reference mirror 6 and a second light beam which is directedalong a sample path SP towards a surface 7 of a sample 8 mounted on asample support stage 9. Light reflected from the reference mirror 6returns along the reference path RP to the beam splitter 5 where itinterferes with light reflected from the sample surface 7 back along thesample path SP. A focussing element 10 a is provided to focus an imageof the region of interference onto a detector 10.

Typically, the interferometer has, for example, a Mirau, Michelson orLinnik configuration.

In this embodiment, the detector 10 has, as shown very diagrammaticallyin FIG. 1, a 2D (two-dimensional) array SA of image sensing elements SE.The array SA images an area of the sample surface 7 falling within thefield of view of the detector 10. Each individual sensing element of the2D sensing array of the detector 10 detects the portion of theinterference pattern falling within the acceptance cone of that elementand resulting from a corresponding surface region or pixel of the areaof the sample surface 7 so that, effectively, the imaged area of thesurface can be considered as a 2D array of surface regions or pixels. Inthis example, the detector 10 is a CCD (Charge Coupled Device) digitalcamera, for example, a Vosskühler GmbH: CCD 1300 CCD digital camera.

A motion controller 11 is provided to effect relative movement betweenthe sample surface 7 and the reference mirror 6 so as to change thedifference in the lengths of the paths travelled by light reflected fromthe reference mirror 6 and light reflected from the sample surface 7. Asshown in FIG. 1, the motion controller 11 is arranged to move thereference mirror 6 along the reference path RP. This is equivalent tomoving the sample surface 7 along a scan path in the Z direction shownin FIG. 1.

The detector 10 is arranged to capture or sense the light intensity (iethe interference pattern) at intervals as the reference mirror 6 ismoved. In this example, the detector captures or senses the lightintensity at intervals corresponding to movement of the reference mirror6 by 75 nm. 2D image or frame data representing the intensity patternfor the field of view of the detector 10 is acquired by the detector 10at each interval.

The intensity of the illumination sensed by one sensing element SE ofthe 2D sensing array (that is the portion of the interference patternprovided by light reflected from the corresponding region or surfacepixel of the sample surface 7 imaged on that sensing element SE) variesas the path length difference changes with movement of the referencemirror 6, resulting in a series of fringes which have a coherence peakat the position along the scan path corresponding to zero path lengthdifference.

FIG. 2 shows a graph of light intensity against position Z illustratingthe change in intensity of the light sensed by a sensing element SE ofthe 2D sensing array of the detector 10 (and thus the interferencefringe region) as the relative positions of the reference mirror 6 andsample surface 7 change and showing a coherence peak. The envelope ofthe intensity distribution is the Fourier transform of the spectraldistribution of spatial frequencies in the broadband source.

As is well known in the art of surface metrology, although the surface 7may be nominally flat, the surface may have some surface form andsurface roughness so that different regions or surface pixels of thesurface have different heights. The position or point along the scanpath at which the coherence peak occurs will be different for surfacepixels at different heights. Accordingly, the surface profile ortopography of an area of a surface imaged by the detector 10 can bedetermined by conducting a measurement operation during which the motioncontroller 11 causes effective movement along the scan path and imagesare captured at intervals by the detector 10, and by then analysing theresults to determine the coherence peaks for each surface region orpixel imaged by the detector 10. Generally, to provide sufficient datafor analysis, the scan path will extend beyond the actual measurementpath, that is the scan path includes run up and run down regions forwhich data is acquired for use in the analysis of the data for theactual measurement path.

As shown in FIG. 1, the data processing and control apparatus 3 hascontrol apparatus 30 for controlling operation of the interferometersystem 2, a data processor 32 for processing data received from theinterferometer system 2 under the control of the control apparatus 30and a user interface 31 for enabling a user or operator to controloperation of the control apparatus (and thus of the surface profilingapparatus) and for enabling the user or operator to be provided with adata output representing the results of processing by the data processor32 of the data acquired during a measurement operation and also forenabling messages such as error messages to be communicated to the user.The user interface generally comprises at least a keyboard 31 a and apointing device 31 b, such as a mouse, and a display 31 d.

The data processor 32 has a data receiver 33 comprising a frame capturer33 a and a frame buffer 33 b for receiving and storing successive framesof measurement data from the detector 10 as the reference mirror 6 isscanned along a scan path and a peak finder 34 for, each surface pixelin the surface area imaged by the detector 10, determining from theframes of measurement data acquired by the detector 10, the positionalong the scan path at which the coherence peak (or a related positionhaving a predetermined relationship to the coherence peak, for example aposition halfway down the coherence peak curve from the actual peak)occurs for that surface pixel. The peak finder 34 is thus arranged todetermine, for each surface pixel of the image surface area, the framein which the coherence peak (or related position) occurs for thatsurface pixel.

The peak data is provided by the peak finder 34 to a topographydeterminer 35 that determines from the peak data provided by the peakfinder 34, the relative heights of the surface pixels of the imagesurface area and thus the surface topography of that surface area andprovides the user with a representation of the surface topography viathe user interface 31. In this example, the topography determiner 35 isarranged to cause the display 31 d of the user interface to display abit-map image representing the surface topography.

The data processor 32 also includes a reference calibrator 36 that isarranged, in accordance with instructions received from a user via theuser interface 31, to conduct a reference calibration procedure todetermine the surface features of the reference mirror 6 (that isfeatures of surface form, surface roughness or texture, including anymarks scratches or the like that the reference mirror 6 may have) so asto acquire reference surface features data.

A reference surface features remover 37 is arranged to subtract thereference surface features data from the surface topography data so asto compensate for the surface features of the reference mirror so thatthe surface topography provided to the user interface 31 is not affectedby any surface features in the reference mirror 6.

One example of a surface profiling apparatus in which the interferometerhas a Mirau configuration will now be described with reference to FIGS.3 to 5 in which FIG. 3 shows an overall functional block diagram of thesurface profiling apparatus, FIG. 4 shows a simplified side elevationalview of the apparatus and FIG. 5 shows a block diagram of computingapparatus suitable for providing the data processing and controlapparatus 3.

Referring to FIGS. 3 and 4, an interferometer I of the broadbandscanning interferometer system 2 has a broadband source 4, typicallycomprising a quartz halogen lamp 4 a coupled by an optical fibre cable 4b to light source optics 4 c which generally comprises, in series, adiffuser, a changeable filter, an aperture stop, a lens, a field stopand a collimating lens that provides an emergent light beam L. Thefilter may be a neutral density filter or a band pass filter, designedto restrict the wavelength range of the light emitted by the broadbandsource 4, such as a Helium-Neon laser line filter designed to pass lighthaving a Helium-Neon laser line wavelength.

Broadband light L is directed via abeam splitter 12 to an objective lensassembly 13 which includes, in addition to an objective lens 14, thebeam splitter 5 and the reference mirror 6.

The beam splitter 5 splits the light beam provided by the beam splitter12 into a first reference beam that is directed along the reference pathRP and a second sample beam that is directed along the sample path SPfrom the interferometer I towards the surface 7 of the sample 8 mountedon the sample support stage 9.

The objective lens assembly 13, and thus the reference mirror 6, ismovable in the Z direction by a Z direction mover 15, in this example apiezoelectric mover, under the control of servo/drive circuitry 15 e ofthe control apparatus 30. The sample support stage 9 is movable in X andY directions by an X mover 16 and a Y mover 17, respectively, to enabledifferent areas of the sample surface 7 to be brought within the fieldof view of the detector 10. Although not shown in FIG. 4, the samplesupport stage may be arranged on a tilting system that enables thesurface of the sample support to be tilted about the Z axis.

As shown schematically in FIGS. 3 and 4, the majority I′ of thecomponents of the interferometer I of the broadband scanninginterferometer system 2 (apart from components of the light source priorto and including the optical fibre cable 4 b) are mounted within ahousing 2 a mounted via a carriage 18 to a Z axis datum column 19.

The optical fibre cable 4 b allows the remainder of the components ofthe broadband source to be provided in a separate light source assembly4′ which, as shown in FIG. 4, can be mounted on a work surface WSadjacent to the remainder of the apparatus so that the heat generatinglight bulb 4 a is remote from the interferometer.

The fact that the components of the light source prior to and includingthe optical fibre cable 4 b are outside the housing 2 a is illustratedin FIG. 3 by showing the broadband source projecting from the housing 2a.

The carriage 18 is coupled via a drive mechanism (not shown) such as aball screw or lead screw drive mechanism to a coarse Z positioner 20 inthe form of a manually operable control or, in this example, a DC motorthat enables the carriage 18 and, thus the interferometer I, to be movedup and down the column 19 in the Z direction to enable theinterferometer to be moved to different scanning start positions.

As shown in FIG. 4, the sample support stage 9 is mounted on a support102 that houses the X and Y movers 16 and 17 and supports the samplesupport stage 9. The X and Y movers 16 and 17 comprise in this exampleDC motors coupled to the sample support stage 9 by appropriateconventional drive mechanisms such as rack and pinion or ball screwdrive mechanisms (not shown).

As shown in FIGS. 3 and 4, each of the Z, X and Y movers is associatedwith a corresponding position sensor 15 a, 16 a and 17 a while thecoarse Z positioner 20 may be associated with a coarse Z positionerposition sensor 20 a.

The control apparatus 30 has a controller 21 that controls overalloperation of the interferometer system 2 and communicates with the userinterface 31, data processor 32, and other parts of the controlapparatus 30 which, in this example, consist of the servo drivecircuitry 15 e and X, Y and Z loggers 22, 23 and 24. The controller 21is also arranged to control the servo/drive circuitry to cause the Zmover 15 to move the objective lens assembly by a distance correspondingto a scan step after each exposure of the 2D image sensor detector 10 soas to capture images at the required intervals. The controller 21 alsoreceives an output from the coarse Z positioner position sensor 20 a, ifprovided.

In the case of the X or Y mover 16 or 17, where the mover is a motor,then the corresponding position sensor may be a linear grating encoder.The dashed lines between the support stage 9 and the X and Y positionsensors 16 a and 17 a in FIG. 3 indicate that the position sensors maysense movement of the support stage 9 directly rather than by signalsderived from the corresponding motor. Where the Z mover 15 is apiezoelectric mover, then the position sensor 15 a may be, for example,an interferometric system, such as a grating system, or an LVDT thatprovides signals representing movement of the objective lens assembly 13relative to the housing 2 a of the interferometer. For example, thehousing of the objective lens assembly 13 may carry a diffractiongrating and a fringe detection interferometric system may be mountedwithin the housing 2 a, providing a count of the fringes to the Z logger24 as the objective lens assembly 13 moves relative to the housing 2 a.As another possibility, a capacitive sensor may be used. As a furtherpossibility a Michelson interferometer (with a corner cube attached tothe housing 13) may be used.

The data processing and control apparatus 30 may be implemented byprogramming computing apparatus, for example a personal computer. FIG. 5shows a simplified block diagram of such computing apparatus. As shown,the computing apparatus has: a processor 25 associated with memory 26(ROM and/or RAM); a mass storage device 27 such as a hard disk drive; aremovable medium drive (RMD) 28 for receiving a removable medium (RM) 29such as a floppy disk, CDROM, DVD or the like; input and output (I/O)controllers 200 for interfacing with the components of the broadbandscanning interferometer system to be controlled by the control apparatus(for example, the Z, X and Y movers 15 to 17, the coarse Z positioner 20and the detector 10) to enable the processor 25 to control operation ofthese component; and the user interface 31 consisting, in this example,of user inputs comprising the keyboard 31 a and the pointing device 31b, and user outputs consisting, in this example, of the display 31 dsuch as a CRT or LCD display and a printer 31 c. The computing apparatusmay also include a communications interface (COMMS INT) 199 such as amodern or network card that enables the computing apparatus tocommunicate with other computing apparatus over a network such as alocal area network (LAN), wide area network (WAN), an Intranet or theInternet. In this example, the data receiver 33 is provided as adedicated frame capture circuit board 230 installed within the computingapparatus.

The processor 25 may be programmed to provide the data processing andcontrol apparatus shown in FIG. 3 by any one or more of the followingways:

-   1. by pre-installing program instructions and any associated data in    a non-volatile portion of the memory 26 or on the mass storage    device 27;-   2. by downloading program instructions and any associated data from    a removable medium 29 received within the removable medium drive 28;-   3. by downloading program instructions and any associated data as a    signal SG supplied from another computing apparatus via the    communications interface 199; and-   4. by user input using the keyboard and, if appropriate, the    pointing device.

The computing apparatus, when programmed by program instructions toprovide the control apparatus 30, enables a measurement operation to becontrolled in accordance with instructions received by a user, and theresulting frame data supplied by the detector 10 to be analysed todetermine the surface profile or topography of the area of the surfaceimaged onto the 2D array of the detector 10.

FIG. 6 shows a block diagram illustrating the functional components ofthe reference calibrator 36.

The reference calibrator 36 comprises a reference calibrator controller50 that controls overall operation of the reference calibrator 36, inparticular controls communication with the user interface 31 via a userinterface communicator 57 enabling command and data to be input to thereference calibrator 36 by the user and enabling the referencecalibrator 36 to output data to the user, for example by displaying thedata on the display 31.

The reference calibrator 36 also has a control apparatus communicator 51for enabling communication between the control apparatus 30 and thereference calibrator 36.

In addition, the reference calibrator 36 has a surface topography datareceiver 52 for receiving surface topography data from the topographydeterminer 35, a surface topography data processor 53 for carrying out anumber of processing operations on the surface topography data and amean surface calculator 54 for determining, for each surface pixel, avalue representing a mean of a number of surface topography data valuesfor that surface pixel determined as a result of a number of calibrationmeasurement operations to be described below.

The mean surface calculator 54 is arranged to store mean surface data ina mean surface data store 59. A reference surface features data storer60 is also provided and is arranged, under the control of the referencecalibrator controller 50, to cause the mean surface data stored in themean surface data store 59 at the end of a calibration procedure to bestored in a reference surface features data store 61, if the userindicates that the calibration is acceptable, or to retrieve previousreference surface features data from a previous reference surfacefeatures data store 62 and store that in the reference surface featuresdata store 61, if the user indicates that the results of the calibrationare not acceptable.

The reference calibrator 36 also includes, in this example, a surfaceroughness indicator calculator 55 for providing an indication of surfaceroughness to give the user an indication as to how close the currentmean surface data is to removing the surface features of the referencemirror, that is an indication as to how well the calibration procedureis proceeding, and a change indicator calculator 56 for calculating avalue representing a change or drift in the surface features of thereference mirror 6 since the last calibration to give the user an ideaas to how frequently it may be necessary to recalibrate the surfaceprofiling apparatus.

Operation of the surface profiling apparatus described above during areference calibration will now be described with the help of FIGS. 7 to9 which show flow charts illustrating steps carried out by the surfaceprofiling apparatus during the reference calibration procedure, FIG. 10which shows a reference calibration interface displayed to the userduring the reference calibration procedure and FIGS. 11 a to 11 c whichshow examples of images displayed to the user by the user interfaceshown in FIG. 10.

When the user selects a reference calibration procedure from, forexample, a selection user interface (not shown) displayed by the controlapparatus 30 on the display 31 d of the user interface, then at step S10in FIG. 7, the control apparatus 30 and the reference calibratorcontroller 50 communicate via the control apparatus communicator 51 toset the image acquisition conditions so that: the zoom is set at ×1(that is, no zoom); the binning (the number of adjacent sensing elementsSE outputs that are added together) is set to ×1, that is no binning;and the neutral density filter is selected. In addition, the referencecalibrator controller 50 ensures that the mean surface data store 59 isempty.

The reference calibrator controller 50 then, at S11 in FIG. 7, causes,via the user interface communicator 57, the display 31 d to display areference calibration user interface screen.

FIG. 10 shows an example of a reference calibration user interfacescreen 70. As shown in FIG. 10, the screen 70 has a Windows styleappearance with a title bar 71 that, although not shown, displays dataidentifying the surface profiling apparatus, and the usual close andminimise buttons 72 and 73 (and optionally also a maximise button 74) inthe top right hand corner. A working area 75 of the referencecalibration user interface screen 70 displays an image window 76 withinwhich images are displayed to the user and a reference calibrationwindow 77.

The reference calibration window 77 has an instruction window 78entitled “artefact set up instructions” that displays user instructionsfor setting up an artefact to be used for the calibration procedure. Inthe interests of simplicity, these instructions are illustrated simplyby a number of parallel lines in FIG. 10.

In this example, the artefact set up instructions instruct the user toplace an artefact in the form of a λ/50 circular coated glass mirrorflat as the sample 8 on the sample support stage 9, then to level theartefact and to adjust the Z height of the interferometer I using thecoarse Z positioner 20 to bring the image into focus so as to make theinterference fringes visible.

The artefact set up instructions 78 also include information provided bythe control apparatus 30 identifying the lens magnification currentlybeing used by the objective lens assembly 13 of the surface profilingapparatus (for example ×10 or ×20) so that the user can calibrate thesurface profiling apparatus for different lens magnifications.

Thus, at S11 in FIG. 7, the reference calibrator controller 50 causes,via the user interface communicator 57, the display 31 d to display thereference calibration user interface screen 70 and prompts the user toset up the artefact, in this case a coated glass mirror flat.

Then, at S12, the reference calibrator controller 50 causes the currentnumber of measurements made and the number of measurements remaining orstill to go to be displayed in respective data display windows 79 and 80of the reference calibration window 77. As this is the firstmeasurement, then the reference calibrator controller 50 will cause, viathe user interface communicator 57, the number “0” to be displayed inthe window 79 and the total number of required measurements to bedisplayed in the window 80. In this example, the total number ofrequired measurements for the reference calibration is preset as, forexample, 8. However, as an option, the window 80 may be a drop downwindow that enables a user to change the number of measurements, forexample to select from two alternatives such as 8 or 12.

The reference calibrator controller 50 then waits at S13 in FIG. 7 forthe user to select a start measurement button 81 of the referencecalibration window 77 using the pointing device 31 b or keyboard 31 a toinstruct a calibration measurement operation to be carried out.

When the user selects the start measurement button 81, then thisinstruction is supplied via the user interface communicator 57 to thereference calibrator controller 50 which instructs the control apparatus30 via the control apparatus communicator 51 to commence a calibrationmeasurement operation at step S14 in FIG. 7.

When the control apparatus 30 receives an instruction to commence acalibration measurement operation, then the controller 21 instructs the2D image sensor detector 10 to commence acquiring images. After eachexposure by the 2D image sensor detector 10 to capture an image, thecontroller 21 requests the servo/drive circuitry to cause the Z mover 15to move the objective lens assembly 13 (and thus the reference mirror 6)by a scan step with the scan step distance and the length of the scanpath (that is the total number of scan steps) being predefined for thereference calibration.

FIG. 9 shows a flow chart illustrating the steps carried out by the dataprocessor 32 to provide the artefact surface topography data.

The detector 10 supplies images or frames of the interference patterncaptured at the required intervals or scan steps along the scan path tothe frame capturer 33 a of the data receiver 33. The captured frames arestored in the frame buffer 33 b of the data receiver 33 in associationwith data identifying the nominal Z position corresponding to that scanstep (and thus that image) determined by the controller 21 from thesignals logged by the Z logger 24 in accordance with the output of the Zposition sensor 15 a. The data receiver 33 thus receives, at step S30 inFIG. 9, for each scan step, frame data representing the intensity valuesensed by each sensing element SE at that scan step.

The peak finder 34 then processes the frame data at step S31 in FIG. 9to determine, for each surface pixel (which corresponds to a sensingelement SE when the binning is ×1) of the artefact the frame and thusthe scan step and Z position at which the intensity value represents thecoherence peak (or a position related to the coherence peak) for thatsurface pixel.

Then, at step S32 FIG. 9, the surface topography determiner 35determines from the peak data provided by the peak finder 34 therelative heights of the coherence peaks (or related positions)determined for each surface pixel to provide artefact surface topographydata.

The peak finder 34 may identify the coherence peak (or position relatedto the coherence peak) and surface topography determiner 35 maydetermine the surface topography by, for example, using any of thetechniques described in U.S. Pat. Nos. 4,340,306 and 4,387,994 ordescribed in International Patent Application number GB03/001067(WO03/078925), the whole contents of which are hereby incorporated byreference.

The surface topography data determined by the topography determiner 35is supplied via the surface topography data receiver 52 to the surfacetopography data processor 53 which, under the control of the referencecalibrator controller 50, processes the artefact surface topography dataat S16 in FIG. 7 to effect, in order: levelling to compensate forsurface tilt; thresholding to remove or modify excessive data values;where data for a surface pixel is missing, replacing or filling in themissing data with data obtained by interpolation from adjacent surfacepixel data or a similar technique; and low pass filtering to remove highfrequency components.

In this example, the levelling procedure is a zero mean levellingprocedure that involves the fitting of a polynomial using a leastsquares fitting procedure to determine a constant representing theaverage surface height and x and y coefficients representing the averagesurface gradient or tilt and then subtracting the determined averagesurface height and the average surface gradient or tilt from the surfacetopography data while the thresholding procedure removes values over 5×the root mean square (RMS) value of the data or truncates any suchvalues to 5× the RMS value.

Then, at S17 the reference calibrator controller 50 causes, via the userinterface communicator 57, the display 31 d to display in the imagewindow 76 (FIG. 10) an image representing the processed artefact surfacetopography data.

FIGS. 11 a and 11 b show images 90 and 91, respectively that may bedisplayed to the user at this stage. Display of the processed artefactsurface topography data to the user enables the user to check theacquired image to see if it is satisfactory. In this case, the image 90shown in FIG. 11 a is generally satisfactory. However, a speck of dust92 is clearly visible in the image shown in FIG. 11 b indicating thatthe calibration measurement operation from which the image data wasderived was unsatisfactory.

Once the user has inspected the displayed image, then the user can electto accept the measurement by selecting an accept button 83 or to rejectit selecting a reject button 84 of the reference calibration window 77shown in FIG. 10. In the present case, if the image displayed is theimage 90 shown in FIG. 11 a then the user should select the acceptbutton 83 whereas if the image displayed is the image shown in 11 b thenuser should select the reject button 84.

The reference calibrator controller 50 waits at S18 in FIG. 7, for theuser to select either the accept button 83 or the reject button 84.

If the user selects the reject button 84 because the image displayedindicates that the measurement is unsatisfactory (because of specks ofdust and the like), then the reference calibrator controller 50 discardsthat processed surface topography data at S19.

If, however, the user selects the accept button 83 then, at S20 in FIG.7, the reference calibrator controller 50 causes the mean surfacecalculator 54 to update the mean surface data stored in the mean surfacedata store 59 using a mean calculation method that retains amplitude asvalues are added.

In this example, the mean surface calculator 54 is arranged to calculatean amplitude-retaining mean by causing the processed surface topographydata for the first calibration measurement operation to be copied to themean surface data store 59 and then by, for each subsequent calibrationmeasurement operation, updating the mean by, for the data representing asurface pixel, adding a proportion of the current processed surfacetopography data for that surface pixel to a proportion of the currentlystored mean surface data value for that surface pixel where the twoproportions add up to one, for example the mean surface calculator 54may be arranged to add 1/n of the value for a surface pixel in thecurrent processed surface topography data to (n−1)/n of the currentlystored mean surface data value for that surface pixel where n is, forexample, eight.

As another possibility, the mean surface calculator 54 may be arrangedto calculate a fixed mean by, for each surface pixel, again copying theprocessed surface topography data for the first calibration measurementoperation to the mean surface data store 59 but then adding ½ of theprocessed surface topography data for the for the second calibrationmeasurement operation to ½ of the currently stored mean surface data,adding ⅓ of the current processed surface topography data for the thirdcalibration measurement operation to ⅔ of the current mean surface data,adding ¼ of the current processed surface topography data for the fourthcalibration measurement operation to ¾ of the currently stored meansurface data and so on until the nth calibration measurement operation.In this case the mean surface calculator 54 includes a calibrationmeasurement operation counter for maintaining a count of the currentnumber accepted calibration measurement operations.

Once the mean surface data has been calculated, then, at S21 in FIG. 8,the reference calibrator controller 50 causes, via the user interfacecommunicator 57, the mean surface data stored in the mean surface datastore 59 to be displayed to the user on the user interface 70 with theimage label 76 a reading: “current mean surface”.

Then, at S22 in FIG. 8, the reference calibrator controller 50 causesthe surface roughness indicator calculator 55 to calculate a surfaceroughness indicator by, for each surface pixel, removing the currentmean surface data stored in the mean surface data store 59 from thecurrent processed surface topography data to produce surface differencedata and then calculating either the root mean square (RMS) of thesurface different data or determining a maximum peak to minimum peakvalue, St for the surface difference data. Alternatively, the surfaceroughness indicator calculator 55 may calculate the surface roughnessindicator by determining the RMS of the current mean surface data.

Then, at S23, the reference calibrator controller 50 causes the surfaceroughness indicator calculated by the surface roughness indicatorcalculator 55 to be displayed in a roughness window 82 of the referencecalibration window 77.

Then, at S24, the reference calibrator 50 checks whether the results ofthe predetermined number n of calibration measurement operations havebeen accepted by the user. If not, then, optionally, at S24 a in FIG. 8,the reference calibrator controller 50 may cause, via the user interfacecommunicator 57, the user interface to display a message prompting theuser to re-position the artefact for the next calibration measurementoperation in accordance with the artefact setup instructions.

When the reference calibration user interface prompts the user tore-position the artefact at S24 a, then the user may move and/or rotatethe artefact in accordance with the artefact set up instructions. Thisre-positioning of the artefact causes the averaging in S20 in FIG. 7 toremove the form of the artefact from the individual measurements.

The reference calibrator controller 50 then increments the measurementsmade number shown in window 79 by 1 and decrements the measurements togo number in window 80 and waits for the user to select the startmeasurement button 81 after re-positioning the artefact to instruct thenext calibration measurement operation.

The reference calibrator then repeats steps S14 to S24 until the answerat step S24 is yes, that is until the user has accepted thepredetermined number n of calibration measurement operations. At thisstage, the image displayed in the image window at step S21 in FIG. 7will represent the final mean surface data. FIG. 11 c shows an example93 of such an image illustrating that surface features of the artefacthave been removed by the averaging procedure.

The reference calibration procedure so far described thus enables a userto inspect an image representing the processed surface topography dataand to discard an image if it is unsatisfactory for some reason, forexample if it shows signs of specks of dust or other dirt, so that onlymeasurements that the user considers are satisfactory are used in thereference calibration procedure.

In addition, the reference calibrator user interface provides the userwith an indication in roughness window 82 in FIG. 10 of surfaceroughness which should provide the user with an indication as to howwell the calibration procedure is doing in removing surface features ofthe artefact because the surface roughness should reach a minimum whenthe surface features of the artefact have been removed so that the onlyremaining surface features are surface features of the reference mirror(that is features of surface form, surface roughness or texture,including any marks scratches or the like). The roughness indicator mayalso alert the user to any problems with the calibration procedure.Thus, for example, if the roughness indicator suddenly increasesmarkedly, then the user will know that a problem has occurred with thecalibration procedure and can immediately select a calibration rejectbutton 87 of the reference calibration window 77 shown in FIG. 10 toabort the calibration procedure.

Assuming that the user has no reason to abort the calibration procedurethen when, at step S24 in FIG. 8, the reference calibrator controller 50determines that the user has accepted n measurements, then, at S25, thereference calibrator controller 50 causes the change indicatorcalculator 56 to calculate a change indicator that provides the userwith drift information relating to changes in time in the surfacefeatures of the reference mirror 6 to give the user an indication as tohow frequently it may be necessary to re-calibrate the reference.

In this example, the change indicator calculator 56 is arranged tocalculate the difference between the current surface roughness indicatorcalculated by the surface roughness indicator 55 and the previous achange window 85 of the reference calibration window 77.

As another possibility, the change indicator calculator 56 may, as shownby the dotted lines in FIG. 6, access the mean surface data store 59 andthe reference surface features data store 60 and calculate thedifference between the mean surface data and previous stored referencesurface features data, determine the root mean square of the differenceand then display that value as an indication of change with time in thesurface features of the reference mirror.

Once the change indicator has been calculated and displayed in thechange window 85 at step S25 then, at step S26, the reference calibratorcontroller 50 waits for input from the user via the user interfacecommunicator 57 indicating whether the user has selected a calibrationaccept button 86 of the reference calibration window 77 to accept theresults of the calibration procedure or has selected the calibrationreject button 87.

The user may use the change indicator displayed in the change window 85and the roughness indicator displayed in the roughness window 82 todetermine whether or not to accept the calibration. For example, if oneor other of these values is very high then the user may consider thatthe calibration is suspect and may reject it. The user may use his ownjudgement to determine whether these indicators are within acceptablelimits or may be provided with guidelines as to acceptable values in theartefact set up instructions.

If the user selects the accept button 86 then, at S28 in FIG. 8, thereference calibrator controller 50 causes the reference surface featuresdata storer 60 to replace the previous reference surface features datain the reference surface features data store 61 with the mean surfacedata stored in the mean surface data store 59.

If, however, the user selects the reject button 87, then at step S27 inFIG. 8, the reference calibrator controller 50 causes the calibration tobe abandoned and causes the reference surface features data storer 60 toaccess the previous reference surface features data from the previousreference surface features data store 62 and set it as the currentreference surface features data in the reference surface features datastore 61. The fact that the previous reference surface features data isnot overwritten means that, if something does go wrong during thecalibration procedure, the previous reference surface features data canbe reinstated.

The above described calibration procedure may be repeated for eachdifferent type of lens of the objective lens assembly available with thesurface profiling apparatus. Once the reference has been calibrated thenthe user may use the surface profiling apparatus to conduct measurementson actual samples.

Operation of the surface profiling apparatus to conduct a measurementoperation on an actual sample will now be described with the aid ofFIGS. 12 and 13 in which FIG. 12 shows a measurement operation userinterface screen 500 that the control apparatus 30 causes to bedisplayed to the user when the user selects to carry out a measurementoperation and FIG. 13 shows a flowchart for illustrating operation ofthe surface profiling apparatus during a measurement operation.

As shown in FIG. 12, the measurement operation user interface screen 500has a Windows style appearance with a minimise button 500 a, a closebutton 500 c, a title bar 500 d and optionally a maximise button 500 b.

The working area of the display screen 500 has a first tabbed window 501having a tab title bar “image” 502 and a second tabbed window 503 havinga tab title bar “measurement set up” 504.

The window 501 is configured to display an image or frame acquired bythe 2D image sensor detector 10 while the window 503 is configured toprovide a user with a number of tools or operational functions to aidthe user in setting up of the scan path for a measurement operation.

The window 503 has a first border 505 labelled “camera” bounding userselectable functions relating to functions of the detector 10, a secondborder 506 labelled “piezo height” encompassing a user selectable optionfor selecting a height for the piezoelectric Z mover, a third border 507labelled “scanning” encompassing user selectable scanning parameters anda fourth border 508 labelled “piezo” encompassing a piezo range scale510, a pointer or slider bar 509 for indicating the current position ofthe Z mover 15, and a measurement range bar 511 indicating the range ofZ movement set for a measurement operation, the measurement range bar511 being associated with scan end markers 511 a and 511 b forindicating the ends of the measurement path. The slider bar or pointer509 is movable by a user to enable the user to change the currentlocation of the Z mover 15.

The piezo height border 506 encompasses a button 512 which, whenselected by a user using the pointing device, causes the controlapparatus 30 to move the Z mover 15 to the mid-point of its operationalrange and to move the slider bar or pointer 509 to the correspondingposition on the scale 510 so that it still shows the current position ofthe Z mover 15, and a window 513 that displays, in micrometres (μm), thecurrent position of the Z mover 15 relative to a nominal zero position,that is the position of the Z mover corresponding to the position set bythe coarse Z positioner 20. The window 513 displays the actual currentposition of the Z mover 15 but is configured to enable a user to overtype the current position using the keyboard to cause the controlapparatus 30 to move the Z mover 15 to the new current positionspecified by the user as an alternative to moving the slider bar orpointer 509.

The scanning border 507 encompasses three buttons labelled “set asfinish”, “set as centre” and “set as start” 524, 523 and 522 that areconfigured to be used in combination with the slider bar or pointer 509to enable a user to identify a selected position of the slider bar orpointer 509 to the control apparatus 30 as a finish, centre and startpositions for a measurement operation. The scanning border 507 alsoencompasses three windows 525, 526 and 527 that are arranged to displaya scan time (in seconds), and scan range and scan start position (bothin micrometres). As in the case of the window 513, the scan range andscan start position windows are configured so that a user can over typethe data to cause the controller to modify the range and start position,as the case may be, in accordance with the data input by the user usingthe keyboard.

The slider bar or pointer 509 and measurement range bar 511 andassociated end markers 511 a and 511 b are configured to provide theuser with a clear visual representation and control over therelationship between the measurement path range represented by themeasurement range bar 511 and the start and end of the measurement pathof the piezoelectric Z mover represented by the end markers 511 a and511 b (the actual scan path of the Z mover 15 may include an initial runup portion and a final run down portion beyond the measurement path toenable frames of data to be acquired to enable data analysis over theentire length of the measurement path). The mid-range button 512 enablesthe user easily to cause the Z mover 15 to move to the middle part ofits range of movement to ensure that the Z mover 15 is operating inmid-range and not at the extremes of its operational range.

The camera border 505 encompasses a zoom drop down menu 530 and abinning drop down menu 531 and 532 for enabling a user to select fromavailable zoom and binning options.

In this example, the window 503 also includes a “start measurement”button 600 for enabling a user to instruct the apparatus to start ameasurement operation once the user is satisfied with the measurementset up parameters. Once the user has instructed the apparatus tocommence a measurement operation by selecting the start button 600, thenas described above, the control apparatus controls the detector 10 andmotion controller 11 to cause images to be captured at scan intervalsalong the scan paths. These captured images are stored in the framebuffer of the data receiver 33 and processed by the peak finder 34 andsurface topography determiner 35 at steps S40, S41 and S42 in FIG. 13.These steps correspond to steps S30 to S32 in FIG. 9 except that, ofcourse, in this case, the measurement operation is being conducted on anactual sample.

Then, at S43 in FIG. 13, the reference surface features remover 37modifies the surface topography data to compensate for the surfacefeatures of the reference mirror by subtracting the reference surfacefeatures data stored in the reference surface features data store 61from the surface topography data. Where the user has not selectedbinning of the image data then the subtraction is conducted on aone-to-pixel basis. Where, however, the user has selected binning, thenthe control apparatus 30 cases the reference surface features remover 37to mimic the effect of binning by adding together adjacent referencesurface features pixel data values to produce binned equivalentreference surface features data values. Thus, for example, if the userselects binning which combines or averages the output of four sensingelements SE (a two by two matrix of sensing elements), then thereference surface features remover 37 combines or averages acorresponding set of four (a two by two matrix) adjacent referencesurface features data pixel values.

Once the reference surface features remover 37 has removed the surfacefeatures of the reference from the surface topography data then thecontrol apparatus 30 causes the display 31 b to display the modifiedsurface topography data to the user in the window 501 as a twodimensional bit map image.

Removal of the surface features of the reference from the surfacetopography data enables the surface topography of the sample to beviewed more easily because it is not modified or distorted by thosesurface features of the reference mirror. In addition to removing formin the reference mirror from the measurement results, the calibrationprocedure also removes the effect on the measurement results of surfaceroughness features including marks and scratches that the referencemirror may have.

In addition to the bit map images described above (examples of which areshown in FIGS. 11 a to 11 c) the surface profiling apparatus may beconfigured to show graphical representations of the change in height orsurface topography along particular or user selectable directions acrossthe surface or may be configured to access software that enables suchgraphical representations to be produced. Removing the surface featuresof the reference mirror from the surface topography data enables suchgraphical representations to represent more accurately the surfaceprofile of the selected cross-section.

In the embodiments described above, the reference calibrator providesthe user with a user interface in which the reference calibration set upinstructions and data input is provided by the reference calibrationwindow 77. It will, of course, be appreciated that the set upinstructions may be provided separately from the data entry windows.Also, the user interface 70 need not necessarily have the configurationshown in FIG. 10, for example, the data input windows and selectionbuttons may be organised differently on the screen. As otherpossibilities, for example, the measurements made and the measurementsto go windows may be replaced with slide bars or level indicators thatincrease and decrease, respectively, with the number of measurementsmade. Similarly, it may be possible to provide a graphical rather thannumeric representation representing the roughness and change indicators,in which case, the user interface may provide markers indicating theacceptable range for the represent change indicators.

Although FIGS. 10 and 12 show separate user interface display windows,it is possible that the reference calibration window and the measurementset up window may be respective different tabbed panes of the same userinterface with the user interface being configured to display the one ofthe tabbed panes selected by the user. Such a user interface may alsoinclude further tabbed panes for enabling, for example, othercalibration procedures to be conducted.

FIG. 14 shows, very schematically, an example of another referencecalibration user interface screen 700 that may be provided by thesurface profiling apparatus to assist the user during the referencecalibration procedure instead of that shown in FIG. 10. The userinterface screen 700 has, again, a Windows style appearance with a titlebar 71 and the usual close and minimise buttons 72 and 73 (andoptionally also a maximise button 74) in the top right hand corner. Aworking area 75 of the reference calibration user interface screen 70displays an image window 76 within which images are displayed to theuser and a reference calibration window 770.

The reference calibration window 770 differs from the referencecalibration window 77 shown in FIG. 10 in that is has, a set up border771 bordering a “lens” display window 772 that displays the lens typemagnification that has previously been selected by the user and a single“measurement done” display window 773 that replaces the windows 79 and80 in FIG. 10 by showing the number of measurements accepted out of thetotal required number of accepted measurements, as illustrated 0 out ofa required total of 8 measurements have been accepted.

The reference calibration window 770 also has a set-up border 774 thatbounds a window showing instructions for a user to set up a “CalibrationStandard” or artefact. As an example, the instructions shown advise theuser to: “Ensure Calibration Standard is levelled. Manually adjust Zstage so that fringes appear across the surface. You will need tomanually adjust the position of the Calibration Standard in XY aftereach measurement.”. The set-up border 774 also bounds a measurement “go”or start button 775 which in practice will normally be coloured greenand a restart button 776 that enables a user to re-zero the mean surfacedata in the mean surface data store 59 to restart a calibrationprocedure.

The reference calibration window 770 also has a calibrate border 777that bounds a “Current Surface Error” window 778 for displaying thesurface roughness indicator and a “Current Datum Error” window 779 fordisplaying the change indicator. The calibrate border 777 also bounds ameasurement accept button 780 labelled with a plus sign for enabling theuser to accept a calibration measurement operation. The measurementaccept button 780 replaces the accept and reject buttons 83 and 84 inFIG. 10. In this case, if the user does not click on or select themeasurement accept button 780 button before again selecting themeasurement start button 775, the results of the calibration measurementoperation will be discarded. The calibrate border 777 also bounds acalibration accept button 781 labelled with a tick sign that is greyedout and unselectable until the required number of measurements have beenaccepted. The calibration accept button 781 781 replaces the accept andreject buttons 86 and 87 in FIG. 10. In this case, not clicking on orselecting the calibration accept button 781 before selecting or clickingon a double arrow labelled button 782 to return, for example, to themeasurement screen shown in FIG. 12, will cause the results of thecalibration to be discarded and the previous reference surfaces featuresdata to be reinstated.

In the above described embodiments, the calibration procedure iseffected for the entirety for the reference mirror. The user may,however, be provided with the option to enable calibration to beconducted over only a part of the reference mirror where the user knowsthat only that part of the reference mirror will be used duringsubsequent measurement operations.

In the embodiments described above, the calibration procedure is startedfrom scratch. The calibration procedure could, however, also becommenced from a previously stored reference topography data or may beused to update or modify previously stored reference form data ratherthan to replace it.

The processing carried out on the artefact surface topography data atS16 in FIG. 7 may be modified. For example, the zero mean levellingprocedure may be replaced by a levelling procedure that simplydetermines, by a least squares fitting procedure the average x and ygradients or tilt of the calibration artefact.

In the embodiments described above, the user interface is a graphicaluser interface. The user interface may, additionally, or alternatively,allow a user to input data and/or instructions in spoken form if theuser interface includes a microphone and speech recognition software.Similarly, the user interface may be configured to provide the artefactset up instructions and other instructions to the user in a spoken oraudible form if the user interface includes a loudspeaker and atext-to-speech converter.

As described above, the user has the option whether or not to acceptboth a calibration measurement operation and the final calibration. Asother possibilities, the user may be provided with this option for onlyone of these or neither.

As described above, the surface topography data processor 53 processesthe surface topography data by effecting, in order: levelling tocompensate for surface tilt; thresholding to remove or modify excessivedata values; where data for a surface pixel is missing, replacing orfilling in the missing data with data obtained by interpolation fromadjacent surface pixel data or a similar technique; and low passfiltering to remove high frequency components. One or more of theseprocesses may be omitted. For example the levelling process may beomitted, particularly if the apparatus facilitates user levelling ofsamples on the support 9. Also, the low pass filtering may be omittedalthough it does have the advantages of removing white noise that may bepresent in the signal from the detector and reducing the effect ofslight movments in the reference mirror due, for example, to thermalchanges. Also, the thresholding may be omitted. The missing data processmay be carried out, for example, only when the data is missing in boththe current mean surface data and the current surface topography data.

In addition one or more of the functional components of the surfaceprofiling apparatus may be located separately, for example, remotelyfrom the others. For example, the data processor, the control apparatus,the user interface and the interferometer system may be providedseparately and linked by communication links.

As described above, the surface profiling apparatus uses a z-axis datum.This may be replaced by a gantry or microscope-style support.

1.-68. (canceled)
 69. Surface profiling apparatus for obtaining surfacetopography data for a surface of a sample, the apparatus comprising: asample support to support the sample; a light director operable todirect light along a sample path towards the sample surface and along areference path towards a reference surface such that light reflected bycorresponding regions of the sample surface and the reference surfaceinterfere; a mover operable to effect relative movement along ameasurement path between the sample surface and the reference surface; asensor operable to sense, for each of a number of regions of the samplesurface, light representing the interference fringes produced by thatsample surface region during said relative movement; a controlleroperable to carry out a measurement operation by causing said mover toeffect said relative movement while said sensor senses light intensityat intervals to provide, for each of the number of regions, a set ofintensity values representing interference fringes produced by thatregion during said relative movement; a data processor operable toprocess the sets of light intensity data to determine from the lightintensity data values associated with each sensed region a positionalong the measurement path at which a predetermined feature occurs inthe light intensity data for that sensed region; and a surfacetopography determiner operable to determine from the positions at whichthe predetermined feature occurs in the light intensity data for thedifferent sensed regions the relative surface heights of the differentsensed regions to provide surface topography data, the apparatus furthercomprising: a reference calibrator operable to calibrate the apparatusto compensate for surface features of the reference surface, thereference calibrator comprising: a user instructor operable to instructa user to conduct a number of calibration measurement operations using acalibration sample having a calibration surface; a user-operablecalibration measurement initiator operable to initiate a calibrationmeasurement operation; a calibration controller operable to cause, inresponse to operation of the calibration measurement initiator,operation of the controller, data processor and surface topographydeterminer to carry out a calibration measurement operation to obtaincalibration surface topography data for the calibration sample; adisplay controller operable to cause a display to display to the user animage representing the calibration surface topography data obtained inthe calibration measurement operation; a user-operable measurementacceptor operable to enable a user either to accept or reject thecalibration surface topography data represented by the displayed image;a mean surface calculator operable to calculate mean surface topographydata using the accepted calibration surface topography data to providereference surface features data when the user has accepted thecalibration surface topography data for said number of calibrationmeasurement operations; and a reference surface features compensatoroperable to adjust surface topography data obtained for a sample surfacein accordance with the reference surface features data.
 70. Apparatusaccording to claim 69, wherein the mean surface calculator is operableto calculate an amplitude-retaining mean each time the user accepts thecalibration surface topography data for a calibration measurementoperation.
 71. Apparatus according to claim 70, wherein the mean surfacecalculator is operable to calculate the mean surface data by setting themean surface data as the calibration surface topography data for a firstaccepted calibration measurement operation and then, for each successiveaccepted calibration measurement operation, adding a first predeterminedproportion of that calibration surface topography data to a secondpredetermined proportion of the current mean surface data.
 72. Apparatusaccording to claim 70, wherein the mean surface calculator is operableto calculate the mean surface data by setting the mean surface data asthe calibration surface topography data for a first accepted calibrationmeasurement operation and then, for each successive accepted calibrationmeasurement operation, adding a first predetermined proportion of thatcalibration surface topography data to a second predetermined proportionof the current mean surface data, wherein the first predeterminedproportion is 1/n and the second predetermined proportion is (n−1)/n.73. Apparatus according to claim 70, wherein the mean surface calculatoris operable to calculate the mean surface data by setting the meansurface data as the calibration surface topography data for a firstaccepted calibration measurement operation and then, for each successiveaccepted calibration measurement operation, adding a first predeterminedproportion of that calibration surface topography data to a secondpredetermined proportion of the current mean surface data, wherein thefirst predetermined proportion is 1/x and the second predeterminedproportion is (x−1)/x where x is a count commencing at 2 for the secondaccepted calibration measurement operation and increasing by 1 with eachsubsequent accepted calibration measurement operation.
 74. Apparatusaccording to claim 69, further comprising a surface roughness determineroperable to determine from the mean surface data and the surfacetopography data a surface roughness indicator and a surface roughnessindicator provider operable to provide the surface roughness indicatorto the user.
 75. Apparatus according to claim 70, further comprising asurface roughness determiner operable to determine, each time acalibration measurement operation is carried out, a surface roughnessindicator using the current mean surface data and surface topographydata and a surface roughness indicator displayer operable to display thesurface roughness indicator to the user.
 76. Apparatus according toclaim 74, wherein the surface roughness determiner is operable todetermine the surface roughness indicator by using data derived bysubtracting the mean surface data from the surface topography data. 77.Apparatus according to claim 74, wherein the surface roughnessdeterminer is operable to determine the surface roughness indicator bydetermining the root mean square of mean surface data or by determiningthe root mean square of data obtained by subtracting the mean surfacedata from the surface topography data.
 78. Apparatus according to claim74, wherein the surface roughness determiner is operable to determinethe surface roughness indicator by determining a peak to peak value ofdata obtained by subtracting the mean surface data from the surfacetopography data.
 79. Apparatus according to claim 69, further comprisinga reference change data determiner operable to determine a referencechange indicator from data representing a change in the referencesurface features data since a previous calibration and a referencechange provider operable to provide the reference change indicator tothe user.
 80. Apparatus according to claim 69, further comprising areference change data determiner operable to determine a referencechange indicator using data representing a change in the referencesurface features data since a previous calibration and a referencechange indicator displayer operable to display the reference changeindicator to the user.
 81. Apparatus according to claim 79, wherein thereference change data determiner is operable to determine the referencechange indicator by subtracting the current reference surface featuresdata from the previous reference surface features data to obtaindifference data and determining the root mean square of the differencedata.
 82. Apparatus according to claim 74, further comprising areference change data determiner operable to determine a referencechange indicator using data representing a change in the surfaceroughness indicator since a previous calibration and a reference changeindicator displayer operable to display the reference change indicatorto the user.
 83. Apparatus according to claim 82, wherein the referencechange data determiner is operable to determine the reference changeindicator by subtracting the current surface roughness indicator fromthe previous surface roughness indicator.
 84. Apparatus according toclaim 69, wherein the reference calibrator has a reference calibrationuser interface provider operable to cause a display to display a userinterface which provides an image display window to display an imagerepresenting surface topography data, the user instructor, auser-selectable start element or button providing the user operablecalibration measurement initiator and a user-selectable accept elementor button providing the user-operable acceptor operable to enable a usereither to accept or reject the calibration surface topography datarepresented by the displayed image.
 85. Apparatus according to claim 74,wherein the reference calibrator has a reference calibration userinterface provider operable to cause a display to display a userinterface which provides an image display window to display an imagerepresenting surface topography data, the user instructor, auser-selectable start element or button providing the user operablecalibration measurement initiator, a user-selectable accept element orbutton providing the user-operable acceptor to enable a user either toaccept or reject the calibration surface topography data represented bythe displayed image, and a display window to display the surfaceroughness indicator.
 86. Apparatus according to claim 79, wherein thereference calibrator has a reference calibration user interface provideroperable to cause a display to display a user interface which providesan image display window to display an image representing surfacetopography data, the user instructor, a user-selectable start element orbutton providing the user operable calibration measurement initiator, auser-selectable accept element or button providing the user-operableacceptor to enable a user either to accept or reject the calibrationsurface topography data represented by the displayed image, and adisplay window to display the reference change indicator.
 87. Apparatusaccording to claim 82, wherein the reference calibrator has a referencecalibration user interface provider operable to cause a display todisplay a user interface which provides an image display window todisplay an image representing surface topography data, the userinstructor, a user-selectable start element or button providing the useroperable calibration measurement initiator, a user-selectable acceptelement or button providing the user-operable acceptor to enable a usereither to accept or reject the calibration surface topography datarepresented by the displayed image, and display windows to display thesurface roughness and reference change indicators.
 88. Apparatusaccording to claim 69, further comprising a mean surface topography datadisplay controller operable to display an image representing the meansurface topography data.
 89. Apparatus according to claim 69, furthercomprising a user-operable calibration acceptor operable to enable auser either to accept or reject the reference surface features data. 90.Apparatus according to claim 89, further comprising an accessor operableto access previous reference surface features data when the user rejectsthe calibration.
 91. Apparatus according to claim 69, wherein thereference calibration controller is operable to cause the controller tocontrol the sensor to sense the entirety of the reference surface duringa calibration measurement operation.
 92. Apparatus according to claim69, wherein the sensor has a plurality of sensing elements and isoperable, in at least one surface measurement mode, to combine or binoutputs from a number of sensing elements representing light sensed froma number of adjacent surface regions during a measurement operation on asample surface to obtain surface topography data for surface areas eachcomprising said number of adjacent surface regions and wherein thereference surface features compensator is operable to combine thereference surface features data for the same number of adjacent surfaceregions before adjusting the surface topography data.
 93. A method ofcalibrating surface profiling apparatus for obtaining surface topographydata for a surface of a sample, the apparatus including: a samplesupport that supports the sample; a light director that directs lightalong a sample path towards the sample surface and along a referencepath towards a reference surface such that light reflected bycorresponding regions of the sample surface and the reference surfaceinterfere; a mover that effects relative movement along a measurementpath between the sample surface and the reference surface; a sensor thatsenses, for each of a number of regions of the sample surface, lightrepresenting the interference fringes produced by that sample surfaceregion during said relative movement; a controller that carries out ameasurement operation by causing said mover to effect said relativemovement while said sensor senses light intensity at intervals toprovide, for each of the number of regions, a set of intensity valuesrepresenting interference fringes produced by that region during saidrelative movement; a data processor that processes the sets of lightintensity data to determine from the light intensity data valuesassociated with each sensed region a position along the measurement pathat which a predetermined feature occurs in the light intensity data forthat sensed region; and a surface topography determiner that determinesfrom the positions at which the predetermined feature occurs in thelight intensity data for the different sensed regions the relativesurface heights of the different sensed regions to provide surfacetopography data, the method comprising the steps of: instructing a userto conduct a number of calibration measurement operations using acalibration sample having a calibration surface; causing, in response tooperation of a user-operable calibration measurement initiator,operation of the controller, data processor and surface topographydeterminer to carry out a calibration measurement operation to obtaincalibration surface topography data for the calibration sample; causinga display to display to the user an image representing the calibrationsurface topography data obtained in the calibration measurementoperation; calculating mean surface topography data using the acceptedcalibration surface topography data to provide reference surfacefeatures data when the user has accepted, using a user-operablemeasurement acceptor, the calibration surface topography data for saidnumber of calibration measurement operations; and storing the meansurface topography data as reference surface features data.
 94. A methodaccording to claim 93, wherein the mean surface calculating stepcalculates an amplitude-retaining mean each time the user accepts thecalibration surface topography data for a calibration measurementoperation.
 95. A method according to claim 94, wherein the mean surfacecalculating step calculates the mean surface data by setting the meansurface data as the calibration surface topography data for a firstaccepted calibration measurement operation and then, for each successiveaccepted calibration measurement operation, adding a first predeterminedproportion of that calibration surface topography data to a secondpredetermined proportion of the current mean surface data.
 96. A methodaccording to claim 94, wherein the mean surface calculating stepcalculates the mean surface data by setting the mean surface data as thecalibration surface topography data for a first accepted calibrationmeasurement operation and then, for each successive accepted calibrationmeasurement operation, adding a first predetermined proportion of thatcalibration surface topography data to a second predetermined proportionof the current mean surface data, wherein the first predeterminedproportion is 1/n and the second predetermined proportion is (n−1)/n.97. A method according to claim 94, wherein the mean surface calculatingstep calculates the mean surface data by setting the mean surface dataas the calibration surface topography data for a first acceptedcalibration measurement operation and then, for each successive acceptedcalibration measurement operation, adding a first predeterminedproportion of that calibration surface topography data to a secondpredetermined proportion of the current mean surface data, wherein thefirst predetermined proportion is 1/x and the second predeterminedproportion is (x−1)/x where x is a count commencing at 2 for the secondaccepted calibration measurement operation and increasing by 1 with eachsubsequent accepted calibration measurement operation.
 98. A methodaccording to claim 93, further comprising the steps of determining fromthe mean surface data and the surface topography data a surfaceroughness indicator and displaying the surface roughness indicator tothe user.
 99. A computer readable medium adapted to instruct a processorto carry out a method in accordance with claim
 93. 100. Surfaceprofiling apparatus for obtaining surface topography data for a surfaceof a sample, the apparatus comprising: sample support means forsupporting the sample; light directing means for directing light along asample path towards the sample surface and along a reference pathtowards a reference surface such that light reflected by correspondingregions of the sample surface and the reference surface interfere;moving means for effecting relative movement along a measurement pathbetween the sample surface and the reference surface; sensing means forsensing, for each of a number of regions of the sample surface, lightrepresenting the interference fringes produced by that sample surfaceregion during said relative movement; control means for carrying out ameasurement operation by causing said moving means to effect saidrelative movement while said sensing means senses light intensity atintervals to provide, for each of the number of regions, a set ofintensity values representing interference fringes produced by thatregion during said relative movement; data processing means forprocessing the sets of light intensity data to determine from the lightintensity data values associated with each sensed region a positionalong the measurement path at which a predetermined feature occurs inthe light intensity data for that sensed region; and surface topographydetermining means for determining from the positions at which thepredetermined feature occurs in the light intensity data for thedifferent sensed regions the relative surface heights of the differentsensed regions to provide surface topography data, the apparatus furthercomprising: reference calibration means for calibrating the apparatus tocompensate for surface features of the reference surface, the referencecalibration means comprising: user instruction means for instructing auser to conduct a number of calibration measurement operations using acalibration sample having a calibration surface; user operablecalibration measurement initiation means for initiating a calibrationmeasurement operation; calibration control means for causing, inresponse to operation of the calibration measurement initiation means,operation of the control means, data processing means and surfacetopography determining means to carry out a calibration measurementoperation to obtain calibration surface topography data for thecalibration sample; display control means for causing a display todisplay to the user an image representing the calibration surfacetopography data obtained in the calibration measurement operation;user-operable measurement acceptance means for enabling a user either toaccept or reject the calibration surface topography data represented bythe displayed image; mean surface calculating means for calculating meansurface topography data using the accepted calibration surfacetopography data to provide reference surface features data when the userhas accepted the calibration surface topography data for said number ofcalibration measurement operations; and reference surface featurescompensating means for adjusting surface topography data obtained for asample surface in accordance with the reference surface features data.101. Surface profiling apparatus for obtaining surface topography datafor a surface of a sample, the apparatus comprising: a sample support tosupport the sample; a light director to direct light along a sample pathtowards the sample surface and along a reference path towards areference surface such that light reflected by corresponding regions ofthe sample surface and the reference surface interfere; a mover toeffect relative movement along a measurement path between the samplesurface and the reference surface; a sensor to sense, for each of anumber of regions of the sample surface, light representing theinterference fringes produced by that sample surface region during saidrelative movement; a controller to carry out a measurement operation bycausing said mover to effect said relative movement while said sensorsenses light intensity at intervals to provide, for each of the numberof regions, a set of intensity values representing interference fringesproduced by that region during said relative movement; a data processorto process the sets of light intensity data to determine from the lightintensity data values associated with each sensed region a positionalong the measurement path at which a predetermined feature occurs inthe light intensity data for that sensed region; and a surfacetopography determiner to determine from the positions at which thepredetermined feature occurs in the light intensity data for thedifferent sensed regions the relative surface heights of the differentsensed regions to provide surface topography data, the apparatus furthercomprising: a reference calibrator to calibrate the apparatus tocompensate for surface features of the reference surface, the referencecalibrator comprising: a user operable calibration measurement initiatorto initiate a calibration; a calibration controller to cause, inresponse to operation of the calibration measurement initiator,operation of the controller, data processor and surface topographydeterminer to carry out a number of calibration measurement operationsto obtain in each calibration measurement operation calibration surfacetopography data for the calibration sample; a surface topography dataprocessor to process the calibration surface topography data obtained inthe calibration measurement operations; a mean surface calculator tocalculate mean surface topography data using the processed calibrationsurface topography data to obtain reference surface features data toenable the reference surface features to be taken into account forsurface topography data obtained in a subsequent measurement operation;a surface roughness determiner to determine from the mean surface dataand the surface topography data a surface roughness indicator; and asurface roughness indicator provider to provide the surface roughnessindicator to the user.
 102. Surface profiling apparatus for obtainingsurface topography data for a surface of a sample, the apparatuscomprising: a sample support to support the sample; a light director todirect light along a sample path towards the sample surface and along areference path towards a reference surface such that light reflected bycorresponding regions of the sample surface and the reference surfaceinterfere; a mover to effect relative movement along a measurement pathbetween the sample surface and the reference surface; a sensor to sense,for each of a number of regions of the sample surface, lightrepresenting the interference fringes produced by that sample surfaceregion during said relative movement; a controller to carry out ameasurement operation by causing said mover to effect said relativemovement while said sensor senses light intensity at intervals toprovide, for each of the number of regions, a set of intensity valuesrepresenting interference fringes produced by that region during saidrelative movement; a data processor to process the sets of lightintensity data to determine from the light intensity data valuesassociated with each sensed region a position along the measurement pathat which a predetermined feature occurs in the light intensity data forthat sensed region; and a surface topography determiner to determinefrom the positions at which the predetermined feature occurs in thelight intensity data for the different sensed regions the relativesurface heights of the different sensed regions to provide surfacetopography data, the apparatus further comprising: a referencecalibrator to calibrate the apparatus to compensate for surface featuresof the reference surface, the reference calibrator comprising: a useroperable calibration measurement initiator to initiate a calibration; acalibration controller to cause, in response to operation of thecalibration measurement initiator, operation of the controller, dataprocessor and surface topography determiner to carry out a number ofcalibration measurement operations to obtain in each calibrationmeasurement operation calibration surface topography data for thecalibration sample; a surface topography data processor to process thecalibration surface topography data obtained in the calibrationmeasurement operations; a mean surface calculator to calculate meansurface topography data using the processed calibration surfacetopography data to obtain reference surface features data to enable thereference surface features to be taken into account for surfacetopography data obtained in a subsequent measurement operation; areference change data determiner operable to determine a referencechange indicator from data representing a change in the referencesurface features data since a previous calibration; and a referencechange provider operable to provide the reference change indicator tothe user.
 103. Surface profiling apparatus for obtaining surfacetopography data for a surface of a sample, the apparatus comprising: asample support to support the sample; a light director to direct lightalong a sample path towards the sample surface and along a referencepath towards a reference surface such that light reflected bycorresponding regions of the sample surface and the reference surfaceinterfere; a mover to effect relative movement along a measurement pathbetween the sample surface and the reference surface; a sensor to sense,for each of a number of regions of the sample surface, lightrepresenting the interference fringes produced by that sample surfaceregion during said relative movement; a controller to carry out ameasurement operation by causing said mover to effect said relativemovement while said sensor senses light intensity at intervals toprovide, for each of the number of regions, a set of intensity valuesrepresenting interference fringes produced by that region during saidrelative movement; a data processor to process the sets of lightintensity data to determine from the light intensity data valuesassociated with each sensed region a position along the measurement pathat which a predetermined feature occurs in the light intensity data forthat sensed region; and a surface topography determiner to determinefrom the positions at which the predetermined feature occurs in thelight intensity data for the different sensed regions the relativesurface heights of the different sensed regions to provide surfacetopography data, the apparatus further comprising: a referencecalibrator to calibrate the apparatus to compensate for surface featuresof the reference surface, the reference calibrator comprising: a useroperable calibration measurement initiator to initiate a calibration; acalibration controller to cause, in response to operation of thecalibration measurement initiator, operation of the controller, dataprocessor and surface topography determiner to carry out a number ofcalibration measurement operations to obtain in each calibrationmeasurement operation calibration surface topography data for thecalibration sample; a surface topography data processor to process thecalibration surface topography data obtained in the calibrationmeasurement operations; a mean surface calculator to calculate meansurface topography data using the processed calibration surfacetopography data to obtain reference surface features data to enable thereference surface features to be taken into account for surfacetopography data obtained in a subsequent measurement operation, whereinthe sensor has a plurality of sensing elements and is operable, in atleast one surface measurement mode, to combine or bin outputs from anumber of sensing elements representing light sensed from a number ofadjacent surface regions during a measurement operation on a samplesurface to obtain surface topography data for surface areas eachcomprising said number of adjacent surface regions and wherein thereference surface features compensator is operable to combine thereference surface features data for the same number of adjacent surfaceregions before adjusting the surface topography data.